This invention relates in general to an automatic gain control circuit for use in a communication system and in particular it relates to an automatic gain control circuit for use in a time assigned speech interpolation system.
A time assigned speech interpolation or TASI system is a high speed switching and transmission system which uses the idle time (i.e. silence) in telephone calls to interpolate the speech of as many as N talkers onto approximately N/2 transmission facilities. One such system is disclosed in the co-pending application of William A. Morgan, Ser. No. 863,902 filed Dec. 23, 1977, now U.S. Pat. No. 4,153,816, and assigned to the assignee of the present invention. In TASI systems such as the one disclosed therein, if the losses in each of the various transmission facilities are not equal, the volume of a talker's speech signal will vary depending upon the facility over which it has traveled. This volume variation can be objectionable to the listening party.
One possible solution to this problem is to provide an automatic gain control circuit for each of the transmission facilities which equalizes the loss on each. A test signal is generated at the far end of the system and transmitted across each facility to the near end. At the near end, the amplitude of the received signal is compared against a reference signal. If the amplitude of a test signal which has been received after traversing a particular facility is below that of the reference signal, the gain of that facility is increased until the test signal amplitude and the reference signal amplitude are equal to one another. If the amplitude of the received test signal is greater than that of the reference signal, the gain of the facility is decreased until the amplitudes are equal. This procedure is periodically employed for each and every transmission facility.
In providing this automatic gain control circuitry the prior art has typically provided a separate automatic gain control circuit for each and every transmission facility. One problem associated with this arrangement is that a separate reference signal and a separate comparison means are employed for each transmission facility. Since the plurality of reference signals and the plurality of comparison means may vary with respect to one another, the loss of the various facilities may therefore not be equalized. Moreover, if the number of transmission facilities increases, the automatic gain control circuity required also increases as does the cost and complexity thereof.
It is an object of the present invention to provide an automatic gain control circuit for a time assigned speech interpolation system which equalizes the loss of the various transmission facilities employed therein.
It is a further object of the present invention to provide an automatic gain control circuit for a plurality of transmission facilities in a TASI system which is simple and inexpensive in its design.